Organic substances capable of transporting electronic charges with positive holes or electrons can be used as organic semiconductors and can be used as materials for organic electronic devices, such as photoreceptors for copiers, optical sensors, organic electroluminescent (EL) devices, organic transistors, organic solar cells, and organic memory devices.
Such materials are typically used in the form of amorphous thin films or polycrystalline thin films. Thin-film transistors composed of amorphous silicon or polycrystalline silicon are widely used as switching elements for use in, for example, liquid crystal display devices and organic EL display devices. However, these transistors composed of silicon are produced with expensive production facilities. Furthermore, such transistors are formed by deposition at high temperatures and thus cannot be formed on plastic substrates having low heat resistance. To solve this problem, organic transistors including channel semiconductor layers composed of organic semiconductors in place of silicon semiconductors are reported.
In general, organic semiconductors have low carrier mobility, compared with that of silicon semiconductors, thereby reducing the response speeds of transistors. This is a problem in practical use. In recent years, organic semiconductors with mobility equivalent to amorphous silicon have been developed. For example, PTLs 1 and 2 describe compounds each having 2,7-substituted [1]benzothieno[3,2-b][1]benzothiophene skeleton (hereinafter, [1]benzothieno[3,2-b][1]benzothiophene is abbreviated as “BTBT”) and state that the compounds have mobility equivalent to or higher than that of amorphous silicon.
However, the mobility is still insufficient to drive high-definition liquid crystal display devices and organic EL devices. Furthermore, even in the case of TFTs produced under the same conditions, the TFTs have large variations in mobility and thus have low performance reliability. Therefore, organic semiconductors are required to have higher mobility and performance stability when used for TFTs.
In recent years, it has been found that liquid crystal phases of liquid crystal substances, which have heretofore been considered as ion conductive substances, have significantly higher mobility than those of amorphous organic semiconductors. This finding has demonstrated that liquid crystal phases are usable as organic semiconductors.
Liquid crystal substances form molecular condensed phases (liquid crystal phases) oriented in a self-organized manner and are positioned as new types of organic semiconductors with high mobility (10−4 cm2/Vs to 1 cm2/Vs). Furthermore, liquid crystal substances have been found to exhibit excellent properties in which orientation defects, in such as domain boundaries and disclination, characteristic of liquid crystals are less likely to form an electrically active level, the properties being not achieved by conventional amorphous organic semiconductor materials or crystalline organic semiconductor materials. In fact, electronic devices including organic semiconductors composed of liquid crystal phases, for example, optical sensors, photoreceptors for copiers, organic EL devices, organic transistors, and organic solar cells, are produced on an experimental basis.
A prominent characteristic of liquid crystal substances is that the control of molecular orientation, which is generally difficult for non-liquid crystal substances, can easily be performed in liquid crystal phases. For example, regarding rod-like liquid crystal substances, when such a liquid crystal substance is injected into the gap between two substrates as in the case of a liquid crystal cell, in general, the liquid crystal molecules tend to be easily oriented in a state in which the long axis of each of the molecules lies in parallel with surfaces of the substrates at a liquid crystal phase temperature. When such a liquid crystal substance is applied to a substrate, the liquid crystal molecules tend to be easily oriented in a state in which the long axis of each of the molecules lies perpendicularly to a surface of the substrate. In the case of using this characteristic, a thin film (crystal thin film) having controlled molecular orientation in a crystal phase is easily produced by lowering the temperature of a liquid crystal thin film oriented at a liquid crystal phase temperature to cause a phase transition to a crystal phase, as in the case of a liquid crystal phase. In ordinary non-liquid crystalline organic materials, it is difficult to achieve the foregoing orientation control.
It has been reported that when a liquid crystal thin film composed of a liquid crystal substance (a thin film in a state of being a liquid crystal phase) is used as a precursor in forming a crystal thin film by taking advantage of the foregoing features, a crystal thin film having excellent crystallinity and flatness can be produced.
Specifically, a uniform film having excellent surface flatness is produced by forming a liquid crystal film at a liquid crystal phase temperature and cooling the resulting liquid crystal film to a crystallization temperature. Thus, liquid crystal substances are materials highly adaptable to organic semiconductors in the fact that liquid crystal substances can be used for electronic devices as organic semiconductor materials in the form of crystal thin films as well as liquid crystal thin films (for example, Non-Patent Literature 1).
In the case where a liquid crystal substance is used as an organic semiconductor, key factors are what kind of crystalline state of a thin film should be formed, in addition to the preparation of the liquid crystal substance with high electron mobility.
Hitherto, various materials have been synthesized as liquid crystal substances. However, most of targets thereof have been limited to nematic liquid crystals to be used as display materials for display devices using optical anisotropy. Thus, a guideline for the molecular design of a liquid crystal substance suitable when the liquid crystal substance is used as an organic semiconductor, in other words, a way of thinking in which the liquid crystal substance may be synthesized, has never been clarified.
In light of the foregoing circumstances, Patent Literature 3 describes a design guideline for the production of a liquid crystal substance with high mobility. Even if such a liquid crystal substance is used, it is still not clear qualitatively and quantitatively what kind of crystalline state of an organic thin film should be produced in order to produce an organic transistor with both of high mobility and high performance stability. There are problems with the development of an organic thin film and an organic transistor with mobility and performance stability.